The invention relates to a method of manufacturing a plastics material, particularly provided with additives, and to an extruder for carrying out the method.
Plastics, such as for example thermoplastics, duroplastics and elastomers, are frequently provided with additives in order to improve their properties. In particular, plastics are reinforced by fibres such as glass fibres, carbon fibres, aramide fibres, natural fibres and plastic fibres. Such fibres are added either as endless fibres, chopped fibres or matting portions to the plastics.
Various methods are known for manufacturing plastics with additives. The invention relates to one of these known possibilities, by means of which the plastic is provided with additives during melting in the extruders. The extruders serving to carry out known methods of this type have various drawbacks. On the one hand there is a lack of homogeneity and impregnation of the plastics or additives. On the other hand added fibres are broken, so that insufficient reinforcement of the plastic is achieved.
The object underlying the invention is to provide a method of manufacturing a reinforced plastic and a corresponding extruder, so that on the one hand homogeneous plastics with additives may be manufactured and on the other hand no appreciable breakage occurs of the fibres serving for reinforcement.
According to this method, the plastic is moved in batches past the pre-plastifying worm in order to disperse and/or feed in additives. This provides the opportunity of treating the plastic and/or the additives in a controlled and intensive manner outside or next to the pre-plastifying worm and to bring them into combination with one another (to impregnate them). According to a further development of the method according to the invention, in order to move the plastic past the pre-plastifying worm, a secondary worm is used. The preferably separate secondary worm has the advantage that it can be designed differently from the pre-plastifying worm and thus can be better adapted to requirements than said pre-plastifying worm.
In a preferred method, fibres serving for reinforcement, in particular endless fibre skeins, are fed to the plastic in the area of the secondary worm. Here the secondary worm affords the possibility of introducing endless fibres into the molten plastic mass and to work them into the mass in such a way that on the one hand they have a maximum possible length and on the other hand are intensively impregnated.
It has proved particularly advantageous to associate with the secondary worm a pressure-free zone, in which fibres, particularly endless fibres, are introduced into the molten plastic mass. Above all, in this way endless fibres can pass around the secondary worm and thus be introduced into the molten plastic mass. The pressure-free zone preferably extends as far as the pre-plastifying worm, ensuring gentle transfer of the endless skeins from the secondary worm to the pre-plastifying worm. In this connection, and also in the following, the term xe2x80x9cpressure-free zonexe2x80x9d is taken to mean a zone in which there exists only sufficient pressure to transport the plastic onwards. In this zone there is however no negative pressure, permanent pressure and/or back pressure. In another method, by virtue of the fact that, in the area of the pre-plastifying worm, additives such as for example, fibres or rovings are fed to the plastic continuously, and in particular in endless form, there results an effective combination of the reinforcements with the plastic. In particular however the additives are prevented from being broken up to the extent that only small particles remain in the plastic.
According to a further development of the method, the additives are fed to the at least partly melted plastic in the area of a pressure-free zone of the pre-plastifying worm, In this way it is on the one hand possible to provide the pre-plastifying worm with an opening, open to the exterior, for endless feed of the fibres or rovings. On the other hand, particularly good and effective impregnation of the fibres or rovings is achieved over their entire length.
In further development of all methods, the fibres or rovings fed in in endless form are chopped, during feeding to the pre-plastifying worm, into rovings or fibres of finite length. In the simplest case, this may be effected at the inner edge of the feed located in the housing for the endless fibres or rovings to the worm. For this purpose the rovings are always chopped at this inner edge of the feed opening in the housing when a worm spiral of the worm passes along under the feed opening. The endless fibres or rovings are in this way chopped in a controlled manner. The length of the fibres or rovings results from the worm geometry, in particular the pitch of the worm spirals of the pre-plastifying worm or secondary worm.
By means of the secondary worm, in conjunction with means for transferring the plastic from the pre-plastifying worm to the secondary worm and back (with additives) a type of secondary area is provided for the plastics which enables the plastic to be treated in a controlled manner, to provide it with additives under the conditions required, and to work the additives into the plastic in the necessary way, particularly impregnating, dispersing and/or mixing.
The secondary worm, extending preferably parallel to the pre-plastifying worm, has its own secondary worm housing, which is connected at points with the plasticising housing. Thus each worm has its own housing, which separates the secondary worm from the pre-plastifying worn. Both worms in this way represent independent constructive units, which may be separately repaired as required.
The means for transferring the plastic, or plastic provided with additives, from the pre-plastifying worm to the secondary worm or vice versa, in a preferred further development of the device, are in the form of transitional openings. The transitional openings are more appropriately disposed in the housing wall of the plasticising housing and of the secondary worm housing, in such a way that, by means of apertures aligned towards one another in the walls of both housings, the plastic (if necessary with the additive) can pass to the secondary worm and then from this point back to the pre-plastifying worm.
In one design of the extruder, the secondary worm housing can have an opening for supply of fibres, particularly endless fibres, to the secondary worm. This ensures controlled feed of the fibres, particularly endless fibres, to the extruder and, at a point where a corresponding design of the secondary worm, fulfils the conditions for non-destructive feed of the fibres to the plastic and intensive impregnation.
Alternatively, it may be envisaged that the secondary worm may be provided with a dispersing worm. This serves to disperse plastic mixtures; the secondary worm reinforcing this procedure as dispersal is carried out more effectively in the area of the secondary worm than in the area of the pre-plastifying worm.
Preferred embodiments given by way of example of extruders and methods for manufacturing plastics according to the invention, will be explained in more detail with reference to the drawing, which shows: